Method and system for pneumatic control for vibrator source element

ABSTRACT

Method, source array and source element that generate seismic waves. The source element includes an enclosure having an opening covered by a piston; a local supply accumulator fluidly communicating with an interior of the enclosure, a pressure of the fluid inside the local supply accumulator being larger than a pressure of the fluid inside the enclosure; a local supply valve located between the local supply accumulator and the enclosure and configured to control a flow of the fluid from the local supply accumulator to the interior of the enclosure; and a controller configured to control the local supply valve such that the pressure inside the enclosure does not fall below a first preset value based upon an ambient pressure of the enclosure while seismic waves are generated.

BACKGROUND

Technical Field

Embodiments of the subject matter disclosed herein generally relate tomethods and systems and, more particularly, to mechanisms and techniquesfor controlling internal pressure of a marine vibratory source elementfor maintaining hydrostatic balance with the ambient pressure.

Discussion of the Background

Reflection seismology is a method of geophysical exploration todetermine the properties of a portion of a subsurface layer in theearth, information that is especially helpful in the oil and gasindustry. Marine reflection seismology is based on the use of acontrolled source that sends energy waves into the earth. By measuringthe time it takes for the reflections to come back to plural receivers,it is possible to estimate the depth and/or composition of the featurescausing such reflections. These features may be associated withsubterranean hydrocarbon deposits.

For marine applications, a seismic survey system 100, as illustrated inFIG. 1, includes a vessel 102 that tows plural streamers 110 (only oneis visible in the figure) and a seismic source 130. Streamer 110 isattached through a lead-in cable (or other cables) 112 to vessel 102,while source 130 is attached through an umbilical 132 to the vessel. Ahead float 114, which floats at the water surface 104, is connectedthrough a cable 116 to a head end 110A of streamer 110, while a tailbuoy 118 is connected, through a similar cable 116, to a tail end 1108of streamer 110. Head float 114 and tail buoy 118 maintain thestreamer's depth and are also provided with GPS (Global PositioningSystem) or other communication equipment 120 for determining thestreamer's position.

In this regard, knowing the exact position of each sensor 122 (only afew are illustrated in FIG. 1 for simplicity) is important whenprocessing the seismic data these sensors record. Thus, vessel 102 isalso provided with GPS 124 and a controller 126 that collects theposition data associated with streamer head and tail ends and also theposition of the vessel and calculates, based on the streamer's knowngeometry, the absolute position of each sensor.

The same happens for source 130. A GPS system 134 is located on float137 for determining the position of the source elements 136. Sourceelements 136 are connected to float 137 to travel at desired depthsbelow the water surface 104. During operation, vessel 102 follows apredetermined path T while source elements (usually air guns) 136 emitseismic waves 140. These waves bounce off the ocean bottom 142 and otherlayer interfaces below the ocean bottom 142 and propagate asreflected/refracted waves 144 that are recorded by sensors 122. Thepositions of both the source element 136 and recording sensor 122 areestimated based on GPS systems 120 and 134 and recorded together withthe seismic data in a storage device 127 onboard the vessel.

A source element may be impulsive (e.g., an air gun) or vibratory. Avibratory source element is described in U.S. patent application Ser.No. 13/415,216 (herein the '216 application), filed on Mar. 8, 2012, andentitled, “Source for Marine Seismic Acquisition and Method,” assignedto the same assignee as the present application, the entire content ofwhich is incorporated herein by reference.

A vibratory source element experiences increased ambient pressure as itsdepth increases. The increase in ambient pressure is approximately 1 barfor every 10 m of added depth. For vibratory source elements with alarge radiating surface (pistons), the resultant force acting on thissurface due to the hydrostatic force can become so great that, in fact,the resultant force exceeds the force capability of the actuator used todrive the piston. If this happens, the seismic source element becomesunable to generate seismic waves. Transient effects, for example seaswells, can also produce localized fluctuations in ambient pressure nearthe source that can also result in significant forces that act on thepiston face. FIG. 6 illustrates an estimate of the variation in ambientpressure for a source located at 25 m depth that might be experiencedduring a seismic survey.

To make best use of the force that can be developed by the sourceelement's actuator, one approach is to counteract (i.e., balance) thestatic forces acting on the pistons so the actuator only provides adynamic force for generating the seismic waves.

Thus, it is desirable to provide systems and methods that balance thehydrostatic force/pressure acting on the source element while beingtowed underwater.

SUMMARY

According to one exemplary embodiment, there is a source element forgenerating seismic waves in water. The source element includes anenclosure having an opening covered by a piston, wherein the piston isconfigured to move relative to the enclosure to generate the seismicwaves; a local supply accumulator fluidly communicating with an interiorof the enclosure, wherein the local supply accumulator stores a fluidthat is also present inside the enclosure, a pressure of the fluidinside the local supply accumulator being larger than a pressure of thefluid inside the enclosure; a local supply valve located between thelocal supply accumulator and the enclosure and configured to control aflow of the fluid from the local supply accumulator to the interior ofthe enclosure; and a controller configured to control the local supplyvalve such that the pressure inside the enclosure does not fall below afirst preset value based upon an ambient pressure of the enclosure whileseismic waves are generated.

According to another embodiment, there is a source sub-array forgenerating seismic waves in water. The source sub-array includes a floatconfigured to float in water and plural source elements suspended fromthe float. A source element includes an enclosure having an openingcovered by a piston, wherein the piston is configured to move relativeto the enclosure to generate the seismic waves, a local supplyaccumulator fluidly communicating with an interior of the enclosure,wherein the local supply accumulator stores a fluid that is also presentinside the enclosure, a pressure of the fluid inside the local supplyaccumulator being larger than a pressure of the fluid inside theenclosure, a local supply valve located between the local supplyaccumulator and the enclosure and configured to control a flow of thefluid from the local supply accumulator to the interior of theenclosure, and a controller configured to control the local supply valvesuch that the pressure inside the enclosure does not fall below anambient pressure of the enclosure while seismic waves are generated.

According to yet another embodiment, there is a source element forgenerating seismic waves in water. The source element includes anenclosure having an opening; a main piston connected to the enclosure toprevent ambient water entering the opening, a movement of the mainpiston generating the seismic waves; a secondary piston located insidethe enclosure and dividing the enclosure into first chamber and secondchamber, wherein the first chamber is fluidly isolated from the secondchamber; an actuation system for actuating the secondary piston insidethe enclosure; and a controller connected to the actuation system andconfigured to increase or decrease a volume of the first chamber bymoving the secondary piston for maintaining a pressure inside the firstchamber balanced with an outside pressure of the enclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate one or more embodiments and,together with the description, explain these embodiments. In thedrawings:

FIG. 1 is a schematic diagram of a seismic acquisition system;

FIGS. 2A-B are schematic diagrams of high- and low-frequency sourceelements;

FIG. 3A is a schematic diagram of a sub-array having a pneumatic controlsystem;

FIG. 3B is another schematic diagram of a sub-array having a pneumaticcontrol system;

FIGS. 4A-C illustrate pneumatic valve states while controlling a sourceelement;

FIG. 5 is a schematic diagram illustrating fluid flows while controllinga source element;

FIG. 6 is a graph illustrating a variation of an ambient pressure withtime while a source element is underwater;

FIG. 7A is a schematic diagram of a pneumatic control system accordingto another embodiment;

FIG. 7B is a schematic diagram of a pneumatic control system having areversible pump according to an embodiment;

FIG. 8 is a schematic diagram of a pneumatic control system according tostill another embodiment;

FIG. 9 is a flowchart of a method for maintaining a pressure balance fora source element;

FIGS. 10A-B illustrate a possible distribution of low- andhigh-frequency source elements in a source array;

FIG. 11 illustrates a multi-component source array;

FIG. 12 illustrates a curved streamer;

FIG. 13 is a flowchart of a method for acquiring seismic data with asource having an optimized piston; and

FIG. 14 is a schematic diagram of a control device for implementingmethods as noted above.

DETAILED DESCRIPTION

The following description of the exemplary embodiments refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. The following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims. The following embodimentsare discussed, for simplicity, with regard to the terminology andstructure of a vibratory source element configured to generate acousticenergy in a marine environment. However, the embodiments to be discussednext are not limited to a marine environment; they may be applied to anytype of source of seismic waves that uses moving pistons, for example,in sources that are raised and lowered and used in fluid filledboreholes for RVSP (reverse vertical seismic profiling) surveys orcross-hole tomography work.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with an embodiment is included in at least oneembodiment of the subject matter disclosed. Thus, the appearance of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout the specification is not necessarily referring to the sameembodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

According to an embodiment, a pneumatic control system for maintaining abalance between a source element's interior pressure and the ambientpressure is configured to have a control loop that adjusts the interiorpressure as the source element's piston moves back and forth. Thus, forsome of the time the pressure inside the source element needs to beincreased, while for other times it has to be decreased. An advantage ofmaintaining hydrostatic balance around the source element pistons isthat this helps to maintain the actuator's position near the center ofits stroke limits. Particularly for source elements' operation thatgenerates low frequencies, where large piston displacements are requiredto radiate useful far-field acoustic signal levels, it is desirable toachieve correct centering of the actuator. Improper actuator centeringmay generate great forces inside the source element's structure, whichin time may result in mechanical failure.

Prior to discussing various pneumatic control systems, a vibratorysource element is introduced, which, for simplicity, is referred toherein as a source element. A source element may have anelectro-magnetic linear actuator system configured to drive a piston (ora pair of pistons). Note that a piston defines not only a rigid materialbut may include a soft material, for example, a diagram. Thus, a pistonis a material that separates an inside of the source element from theambient. However, it is possible to have a hydraulic, pneumatic,magnetostrictive or piezoelectric actuator or other appropriatemechanisms instead of the electro-magnetic actuator. A source elementmay be driven by an appropriate pilot signal. Plural source elements maybe located together to form a source sub-array. One or more sourcesub-arrays form a source array. A vessel is configured to tow a sourcearray. A pilot signal is designed as a source array target signal suchthat the total array far-field output follows a desired target powerspectrum. A drive signal derived from the pilot signal is applied toeach source element. A pilot signal may have any shape, e.g.,pseudo-random, cosine or sine, increasing or decreasing frequency, etc.

According to the embodiment illustrated in FIG. 2A, a source element 200has a housing 220 that, together with pistons 230 and 232, enclose anactuator system 240 and separate it from the ambient 250, which might bewater. Although FIG. 2A shows two movable pistons 230 and 232, note thata source element may have any number of pistons, e.g., one piston ormore than two.

Housing 220 may be configured as a single enclosure as illustrated inFIG. 2A and have first and second openings 222 and 224 configured to beclosed by pistons 230 and 232. However, in another embodiment 201illustrated in FIG. 2B, housing 220 may include two enclosures 220A and220B rigidly connected to each other by a member 202. A single actuatorsystem 240 may be configured to simultaneously drive pistons 230 and 232in opposite directions to generate seismic waves, as illustrated in FIG.2A. Two actuator systems 240A and 240B may be used in the embodiment ofFIG. 2B. In one application, pistons 230 and 232 are rigid, i.e., madeof a rigid material, and reinforced, as will be discussed later, withrigid ribs 234. Actuator system 240 may include one or more individualelectro-magnetic actuators 242 and 244. Other types of actuators may beused. Irrespective of how many individual actuators are used in sourceelement 200 or 201, the actuators are provided in pairs configured toact simultaneously in opposite directions on corresponding pistons inorder to prevent source element “rocking” motion. Note that it isundesirable to “rock” the source element when generating waves becausethe source element's position should follow a predetermined path whentowed in water.

The size and configuration of the housing, pistons and actuator systemdepend on the source element's acoustic output. For example, ahigh-frequency source element (as illustrated in FIG. 2A) has smallersizes than a low-frequency source element (as illustrated in FIG. 2B).In one embodiment, the high-frequency source element's housing length isabout 1.5 m and its diameter is about 450 mm. Total housing length ofthe low-frequency source element is about 3 m and its diameter is about900 mm. Thus, in one application, the low-frequency source element issubstantially double the size of the high-frequency source element.

Actuator system 240 may be attached to housing 220 by an attachment 248(e.g., a wall or a bracket). Various other components describedelsewhere are illustrated in FIGS. 2A and 2B. Such components mayinclude a sealing mechanism 260 provided between the pistons and thehousing, a pressure regulation mechanism 285 or 285A and 285B configuredto balance the external pressure of the ambient 250 with a pressure of afluid 273 enclosed by housing 220 (enclosed fluid 273 may be air orother gases or mixtures of gases), one or more shafts (280 and 282) perpiston to transmit the actuation motion from the actuation system 240 topistons 230 and 232, a guiding system 290 for the shafts, a coolingsystem 294 to transfer heat from the actuator system 270 to ambient 250,one or more local control devices 270, 270A, 270B to coordinate themovement of these elements, etc.

A pneumatic system for maintaining hydrostatic balance in one or moresource elements is now discussed with regard to FIG. 3A, whichillustrates a single sub-array 302 of a source array 300. Sub-array 302includes a float 304 that floats at water surface 306. In oneapplication, float 304 may be configured to float below the watersurface. From float 304, plural source elements 310 a-d are suspendedthrough corresponding cables 312 a-d, e.g. cables, ropes, chains and/orstretchable linkage. The number of source elements may vary depending onsurvey characteristics. FIG. 3A shows four source elements, but thisnumber is exemplary and should not be construed to limit the invention.

Pneumatic system 320 includes a pressure supply mechanism 322 and apressure relief mechanism 324. Pressure supply mechanism 322 is taskedwith moving a fluid from the vessel (in this embodiment, however, it ispossible, as discussed later, to have the fluid supplied from a localaccumulator, thus, autonomous from the vessel) to each individual sourceelement, while the pressure relief mechanism is tasked with removing thefluid from each source element. The supply and relief of the fluid mayhappen in an alternative sequence, as controlled by a control device tobe discussed later. Both mechanisms act to maintain the hydrostaticbalance between the source elements' external and internal pressures.Each mechanism is now discussed in detail.

Pressure supply mechanism 322 includes a fluid supply line 326 (e.g., ahose, conduit, etc.) that takes compressed fluid from the vessel anddelivers it to the sub-array. The fluid may be air, dry air, or anothergas, for example, nitrogen. The fluid may be transferred from the vesselat a high pressure. The fluid is compressed onboard the vessel by a maincompressor (not shown) or another similar device. Supply line length Lcan be in the range of 500 to 1,000 m, in order to position the sourcescloser to the streamer whose receivers are positioned well back of thevessel to avoid noise associated with vessel motion/propulsion.

Because of supply line length, pressure drop and transient responsebecome significant issues. Another significant issue for such aconfiguration is the large number of source elements. For example, amodern source array may include dozens of source elements, eachrequiring pressurized fluid. Further complicating the picture are thelocalized ambient pressure perturbations due to wave action, and seaswells, and the need for each source element to have the pressurizedfluid as quickly as possible to balance the outside pressure. Otherhydrodynamic forces due to towing may also act upon the piston face andneed to be counter balanced. Therefore, the volume of fluid necessaryduring source array operation is significant. Supplying pressurizedfluid from the vessel directly to each source element is slow, whichresults in unsatisfactory pressure balance. Because traditionalpneumatic systems cannot quickly supply the fluid at high pressure oversuch long distances and to so many source elements, this application'sinventors have proposed the following novel features.

To resolve the high-pressure requirement, a sub-array global accumulator328 is located on each sub-array. This global accumulator (note that theterm “global” applies to a given sub-array and not to the entire sourcearray) stores the fluid under pressure and distributes it to each sourceelement in the sub-array as needed. This configuration greatly reducesthe distance between the supply and the source element, e.g., to lessthan 30 m. Using some exemplary numbers, in one application, if thefluid's pressure is 100 bar when leaving the vessel's compressor, thefluid's pressure in the accumulator 328 should also be around 100 bar. Apressure-reducing/pressure regulator valve 330 (e.g., passive types likespring/diaphragm activated or electrically driven solenoid,proportional, poppet, etc.) ensures that the pressurized fluid isdelivered at a lower pressure to the seismic source elements. Eachsource element 310 a has its own local supply accumulator 332 aconnected to pressure-reducing valve 330. The fluid's pressure in thelocal supply accumulator may be between 6 and 10 bars. Thus, thepressure-reducing valve/pressure regulator valve 330 may be configuredto reduce the pressure by a factor of ten. Other values are possible.

Considering that each source element 310 a is a twin driver asillustrated in FIGS. 2A-B, the pressurized fluid is distributed from thelocal supply accumulator 332 a via corresponding local valves (e.g.,pneumatic valves) 334 a-1 and 334 a-2 to the twin drivers. This highpressure is injected in the source element's interior 336 a, on one sideof each piston 338 a, to counterbalance the outside pressure 340 thatacts on the other side of the piston. Note that FIG. 3A is a schematicfigure, not at scale and not intended to show the exact shape, positionand size of the pressure mechanism or seismic source array. Should therebe an increase in ambient pressure which might cause the pistons to moveinward, pressurized fluid is delivered inside the source element at apressure higher than the ambient pressure, thus helping the pistons moveoutwardly relative to the source element housing to keep the piston andactuator centered within its range of travel and to counteract theincrease in ambient pressure. Likewise, should there be a decrease inambient pressure which might cause the pistons to move outward, fluid isvented from inside the source element at a pressure lower than theambient pressure, thus helping the pistons move inwardly relative to thesource element housing to counteract the decrease in ambient pressure.Details about the control mechanism for determining when and how muchpressurized fluid to allow inside the source element are discussedlater.

When the pistons need to move inwardly, the volume of the source elementdecreases, thus generating increased pressure inside it. The pressurerelief mechanism 324 is responsible for preventing/reducing the pressureincrease, as is now discussed. Pressure relief mechanism 324 may uselocal valves 334 a-1 and 334 a-2 or different pairs (if a twin driver isconsidered) of valves for removing fluid from inside the source element.In one application, local valves 334 a-1 and 334 a-2 are three-wayvalves as illustrated in FIGS. 4A-C, that can be a proportional spoolvalve type, a solenoid valve or comprised of poppet valves. However,local valves 334 a-1 and 334 a-2 may be plural single valves, forexample, each valve 334 a-1 and/or 334 a-2 may include a set of singlevalves connected in parallel and driven by a same signal. This setup maybe advantageous for increasing the flow rate. FIG. 4A shows the localsupply accumulator 332 a being in fluid communication with an input 342a of the source element when local valve 334 a-1 is in state A. At thesame time, a local vent accumulator 344 a (e.g., one for each sourceelement or one for the entire sub-array) and an output 346 a of thesource element are shut, i.e., not in fluid communication with eachother. FIG. 4B illustrates the same setup with the difference that localvalve 334 a-1 is now in state B, i.e., blocking local supply accumulator332 a and input 342 a and fluidly communicating local vent accumulator344 a with output 346 a. FIG. 4C illustrates the same setup with thedifference that local valve 334 a-1 is now in state C, i.e., blockinglocal supply accumulator 332 a from communication with input 3422 a andblocking local vent accumulator 344 a from communication with output 346a. State C coincides to the case where the ambient and internal housingpressures have been adequately equalized and no action is required.Other types of valves and/or other arrangements may be used as will beappreciated by those skilled in the art as long as the same control ofthe fluid is achieved. Pressure of the local vent accumulator is desiredto be around 1 bar, but other values may also be used.

The local vent accumulators may be connected to a common line 348 asillustrated in FIG. 3A, and the common line is connected to a vent line350 that directly communicates with the atmosphere above the watersurface 306. Vent line 350 may be attached to float 304 or it may haveits own float (not shown) for maintaining one of its ends in fluidcommunication with the atmosphere. The pressure relief mechanism is thusable to remove the fluid from inside the source elements (acting similarto a vacuum pump) and to expel that fluid in the atmosphere. In oneapplication, if it is preferred to exhaust the housing fluid at depth,rather than running a vent line to the atmosphere to avoid lineentanglements, the inlet of a pneumatic pump equipped with a pressureregulator mechanism could be attached to common line 348 to helpmaintain low pressure in the local vent accumulators. The exhaust portof the pneumatic pump could be connected to a short exhaust hoseequipped with a check valve to vent the exhausted air at or aboveambient pressure.

FIG. 3A also shows that each cable 312 a-d is attached to correspondingplates 352 a-d that offer mechanical support for the source elements,local supply accumulators, local vent accumulators, common lines, etc.FIG. 3A schematically illustrates the plates and their relations to theother elements.

Pressure mechanism 320 may also include pressure sensors distributed atvarious locations, for example, a pressure sensor 360 a inside thesource element, a pressure sensor 362 outside the source element tomeasure the ambient pressure, a pressure sensor 364 a inside localsupply accumulator 332 a, a pressure sensor 366 a inside local ventaccumulator 344 a, and/or a pressure sensor 368 inside global supplyaccumulator 328. A position sensor 370 a may be located inside eachsource element for measuring and/or estimating a position of the pistonrelative to the source element housing. Either the pressure or positionmeasurements (or another appropriate quantity) may be used in a feedbackloop control for balancing the hydrostatic pressure.

According to a variant embodiment illustrated in FIG. 3B, a vacuum pump380 may be connected to common line 348 for removing the low pressureair from the source elements. Vacuum pump 380 may have a vent line 382that discharges the air directly to the ambient. Thus, vent line 350shown in FIG. 3A is not necessary. Vacuum pump 380 may be actuated by apneumatic motor 384. Pneumatic motor 384 may be connected to accumulator328 and is driven by the high pressure air from this accumulator. Thevacuum pump may be, for example, a dry scroll pump. The exhaust of thepneumatic motor may be connected through conduit 386 to local supplyaccumulators 332 a-d to fill them with air to maintain the hydrostaticbalance. In one application, the pneumatic motor may work in parallelwith pressure-reducing valve 330, and a pressure relief valve 388 may beused to keep the supply pressure from getting too high. Thus, accordingto this embodiment, there is no need for a vent hose to the surface andinstead air could be exhausted at the same depth as the source.

In one embodiment, as illustrated in FIG. 5, a source sub-arrayincludes, besides the elements illustrated in FIG. 3A or 3B (andreproduced in FIG. 5), a controller 502. Controller 502 may be locatedon the sub-array to act as a local controller, or on the vessel to actas a global controller, or it may be distributed between the sub-arrayand the vessel. Controller 502 is electrically connected topressure-reducing valve 330, local valve 334 a-1 (also local valve 334a-2, but for simplicity, this case is not illustrated because it behavessimilarly to valve 334 a-1), inside pressure sensor 336 a and outsidepressure sensor 362. Controller 502 is configured to receive, with agiven frequency (preferably less than 2 Hz for not interfering with thepistons' movement), pressure measurements from the inside and outsidepressure sensors. In one application, if a difference between the twopressures is greater than a pre-established threshold, e.g., insidepressure is less than outside pressure, controller 502 controls localvalve 334 a-1 to release more pressurized fluid from local supplyaccumulator 332 a into the inside of source element 310 a. If theopposite situation is true, i.e., outside pressure is less than insidepressure, controller 502 changes the state of valve 334 a-2 to shutlocal supply accumulator 332 a and to fluidly connect the inside of thesource element with the local vent accumulator 344 a to reduce insidepressure. Because inside pressure changes continuously during pistonoperation, and because outside pressure also may change in time (due toswells, changing source element depth, temperature change, etc.) asillustrated in FIG. 6, controller 502 needs to continuously monitorpressure changes and quickly adjust inside pressure. Continuousmonitoring implies that a comparison between internal and externalpressures is performed with certain regularity, e.g., every 0.5 s ormore frequently. Quick adjustment of internal pressure is achieved byhaving the fluid reservoirs close to each source element (i.e., thelocal supply accumulator and local vent accumulator). Note that in oneapplication the same may be achieved without the presence of globalaccumulator 328. Controller 502 may also coordinate the opening andclosing of valve 330 for supplying pressurized fluid from globalaccumulator to local accumulators. If valve 330 is a proportional valve,controller 502 may control the metering area of valve 330 so as tosmoothly vary the fluid flow rate. In one application, each sourceelement has its own controller 502. However, in another application,controller 502 controls all of a given sub-array's source elements. Instill another application, controller 502 controls all the sourceelements of the source array.

Comparing the external and internal pressures is one possible approach(loop) for controlling the source elements' hydrostatic balance. Anotherpossibility is to measure the piston's position relative to the sourceelement housing and then subtract off the displacement contribution dueto the sweep (or some fraction of the displacement contribution due tothe sweep, for example, about 90%), and to use this result as a feedbacksignal to controller 502. Note that the displacement contribution due topiston sweep may be calculated, based on a theoretical model, ormeasured during a dry test prior to deploying the seismic source elementin water. The controller driving the local valves can use an averaged orlow-pass filtered piston displacement signal as a feedback signal tokeep the actuator centered during operation. Other closed loop controlschemes are possible, such as schemes that have a closed loop bandwidththat falls below the sweep frequency bandwidth of interest so thepneumatic control loop ignores rapid piston motion due to sweeping.

Various modifications may be envisioned to the embodiments illustratedin FIGS. 3A-B that still achieve source element pressure balance. Forexample, instead of having vent line 350 communicating with theatmosphere, it is possible to extend this line back to the vessel andform a closed pneumatic circuit so the fluid is returned to the vesselcompressor to be recirculated. To help move the fluid back to the ship,one or more pumps (not illustrated) may be added to the sub-array or toeach source element. In another application, the source element housingincludes a chamber 510, as illustrated in FIG. 5, that houses theelectronics, and it is isolated from the interior 512 of the sourceelement. Chamber 510's pressure may be at atmospheric pressure. In thisembodiment, the fluid from interior 512 (which has a greater pressurethan that inside chamber 510) may be quickly released through aninternal valve 514 to chamber 510. In still another embodiment, chamber510 is made to communicate with valve 334 a-1 for venting the fluidoutside.

In another embodiment illustrated in FIG. 7A, a source sub-array 700 hasplural source elements 702, one of which is illustrated for simplicity.In this embodiment, the hydrostatic balance between source element 702'sinside and outside is achieved by a movable internal piston that adjustsits position to control the inside pressure. More specifically, FIG. 7Ashows a housing 704 having one end 706 closed by an external piston 708.Movement of piston 708 generates the desired seismic waves. FIG. 7Aschematically shows a shaft 710 attached between piston 708 and actuator712. As noted previously, actuator 712 may be attached by a bracket 714or any equivalent structure to housing 704. However, different from theembodiments illustrated in FIGS. 2A-B, an internal piston 716 iscompletely provided inside housing 704 so it divides housing 704'sinside into first chamber 718A and second chamber 718B. A different,secondary actuator system 720 is attached to housing 704 and configuredto actuate internal piston 716 through one or more shafts 722. Formaintaining the pressure inside first chamber 718A, a sealing mechanism724 is distributed between internal piston 716 and housing 704.

A controller 726, which can be located inside or outside the sourceelement, on the vessel, or in any other combination thereof, isconfigured to coordinate main actuator 712 and secondary actuator 720 sothat when a pressure inside first chamber 718A increases more than apredetermined threshold over outside pressure, internal piston 716 movesto increase a volume of first chamber 718A to decrease the pressure inthis chamber. Controller 726 is also configured to move the internalpiston in the opposite direction if the pressure inside first chamber718A decreases. Controller 726 may be connected to pressure sensorssimilar to controller 502 in FIG. 5 for controlling a movement of thetwo pistons. Alternatively, controller 726 may achieve pressure balanceby monitoring a position of piston 708, as is also discussed above withregard to FIG. 5.

Similar to the embodiment illustrated in FIG. 3A, a local supplyaccumulator 730 may be located on or next to source element 702 tosupply pressurized fluid inside first chamber 718A. A valve 732 controlsthe pressurized fluid's inflow inside first chamber 718A. A local ventaccumulator 734 may also be located on or next to the source element forremoving the fluid from inside first chamber 718A. A corresponding valve736 controls the fluid's outflow from first chamber 718A. Alternatively,instead of having local vent accumulator 734, a vent line may be fluidlyconnected to the atmosphere for venting out the fluid from first chamber718A. If a vent line is used, a fan or a pump (not shown) may also beused to control the fluid's outflow. Note that controller 726 isconnected to both valves 732 and 736 and also to the pump, if one ispresent, to control the pressure balance inside and outside firstchamber 718A.

In one application, local supply accumulator 730 is not connected to thevessel or any other fluid supply. In other words, the local supplyaccumulator is an autonomous unit, similar to a scuba diving unit, whichcontains the necessary fluid under pressure. However, in anotherapplication, it is possible to connect the local supply accumulator 730to fluid supply on the vessel. If the local supply accumulator 730 isautonomous, the local vent accumulator or vent line may also beautonomous, i.e., they are not connected to the vessel. In this case,source element 702 is configured to function without pneumaticassistance from the vessel. Note that in one application, internalpiston 716 is configured to adjust the pressure inside first chamber718A to account only for swells.

A similar source element is illustrated in FIG. 7B. However, this sourceelement does not use an internal piston 716 and associated actuatorsystem 720 for balancing the inside pressure with the ambient pressure,but rather uses a pump 752 connected to a reservoir 754 for achievingthe same function. Pump 752 is configured to act either as an aircompressor or as a vacuum pump. Controller 726 controls pump 752 so thatair is removed from source element 750 when the hydrostatic pressuredecreases. When the hydrostatic pressure increases, pump 752 reversesits function and adds air to the source element. This could be a closedsystem with no hoses to the surface or to the vessel. Pump 752 may be avane, scroll or diaphragm pump or a piston pump. In one application,instead of reversing the function of the pump, a four-way valve may beused to reverse the inlet and outlet lines of the pump. Note that localsupply accumulator 730 and local supply valve 732 are optional for thisembodiment.

FIG. 8 illustrates another embodiment similar to that discussed withreference to FIG. 7A, but having a bladder system instead of an internalpiston. More specifically, FIG. 8 shows a rigid enclosure 840 attachedto the housing 804 and forming a second chamber 818B. A bladder system842 is located inside rigid enclosure 840 and sized to release or absorbenough fluid into first chamber 818A to compensate for pressurevariation produced by swells. Bladder system 842 fluidly communicateswith first chamber 818A through a passage 844. Passage 844 may be sizedor contain an orifice, for example, to balance pressure variation due toswells (below 4 Hz) with the minimum pressure loss and to ensure thefunction of actuator 812 for operational frequencies (e.g., between 5 Hzand 25 Hz for a low-frequency source element). Rigid enclosure 840 mayhave another passage 846 that communicates with the ambient (i.e.,seawater) such that the ambient pressure acts directly on the bladderexterior to help equalize the ambient pressure and interior housingpressure. Passage 846 may be sized so as to respond to low frequencychanges in ambient pressure and not so large as to create a significantacoustic leakage pathway for sound produced by the sweep.

The embodiment illustrated in FIG. 8 may have an autonomous local supplyaccumulator 830, i.e., not connected to the vessel's fluid supply.However, in one application, similar to the embodiment illustrated inFIG. 7A, the local supply accumulator 830 may in fact be connected tothe vessel's fluid supply. In one application, bladder 842 may bedesigned to have a volume twice the volume required to balance thepressure inside first chamber 818A. A local vent accumulator or ventline 834 may be connected to first chamber 818A for venting out theexcess pressure inside the first chamber. Controller 826's functionalitymay be limited to controlling only actuator 812 and/or local valves 832and 836. In one embodiment, vent line 834 is connected to a pump 835that is configured to vent out the fluid from the housing. In thisapplication, vent line 834 does not need to extend to the water surface.In another application, if the pump 835 is present, no vent line 834 maybe needed. Pump 835 may be attached to an outside of enclosure 804. Thepump with the above-noted configurations may also be implemented in theother embodiments, e.g., the embodiment of FIG. 5.

The following configuration of the controller may apply to any of theabove-noted embodiments. The controller may be configured to control thelocal supply valve such that the pressure inside the enclosure does notfall significantly below the ambient pressure while seismic waves aregenerated. The controller may also be configured to control the localvent valve so that the pressure inside the enclosure does notsignificantly exceed the ambient pressure while seismic waves aregenerated. The controller may be programmed to read from storage device(e.g., a memory) first and second thresholds such that the pressureinside the enclosure does not fall more than the first thresholdrelative to the ambient pressure, and the pressure inside the enclosuredoes not exceed the ambient pressure by more than the second threshold.In one application, the first and second thresholds are equal.

A method for maintaining pressure balance outside and inside a seismicsource is now discussed with regard to FIG. 9. In step 900, inside andoutside pressures are received at a controller. Alternatively, a pistonposition is received at the controller. In step 902 the two pressuresare compared or the piston position is compared to a given chart thatillustrates the piston position in time. If the external pressure ishigher than the internal pressure by a given value, the controlleractivates in step 904 a valve to supply fluid from a local supplyaccumulator to an inside of the source element. If the oppositecondition is true, the controller activates in step 906 the same valveor another valve to vent fluid out from inside the source element. Thesesteps may be adapted to control the valves based on the piston positionrelative to the given chart. The supplying or venting of fluid mayhappen for a predetermined period of time. In one application, supplyingor venting may happen until a new measurement is performed in step 908.In step 910, the contribution of the sweep to the housing interiorpressure is removed from the output of step 908, and then the processreturns to step 902. In general, for improved efficiency, a housingair-spring resonance is employed in the source element design. Thetrapped fluid inside the housing acts like a spring that in combinationwith the combined mass loading due to the driven structure mass andradiation mass creates a resonance effect, typically in the midrange ofthe sweep range. So it is usually desired to ignore the sweepcontribution to the housing pressure variation so as to not defeat thehousing air-spring.

When implemented in an actual seismic survey system, a seismic sourcearray 1200 having the source elements discussed with reference to FIGS.3, 7 and 8 may have, as illustrated in FIG. 10A, two high-frequencysub-arrays 1002 and a single low-frequency sub-array 1004. Eachsub-array may have plural source elements as discussed above. In oneapplication, the high-frequency sub-arrays 1002 are towed at a depth ofabout 5 m, while the low-frequency sub-array 1004 is towed at a depth ofabout 25 m.

A side view of a marine acquisition system 1006 that includes seismicsources having pistons shaped and configured as discussed above isillustrated in FIG. 10B. System 1006 includes a towing vessel 1008 thattows the seismic array 1000. Seismic array 1000 may include, asdiscussed with regard to FIG. 10A, one or more high-frequency sub-arrays1002 positioned at a depth H1 and one or more low-frequency sub-arrays1004 positioned at a depth H2, where H2 is deeper than H1. Depthcontrollers 1010 may be located on or next to each sub-array formaintaining a desired depth. Umbilicals 1012 connect each sub-array tovessel 1008. An umbilical may include a strength member, command anddata capabilities, electrical power, and pneumatic air supply.

A mechanical interface 1012 connects corresponding umbilical componentsto a pneumatic supply system 1014, a power supply system 1016, and acommand and control device 1018. Command and control device 1018 mayinclude a processing unit, as described later, that is capable toreceive and process seismic data for imagining the surveyed subsurface.Command and control device 1018 may also be configured to control atrajectory of the seismic source, adjust its trajectory and control theshooting of the source elements. Command and control device 1018 mayinteract with the vessel's navigation system.

Although FIG. 10B shows each sub-array having a horizontal distribution,note that a multi-level source may be used instead of source array 1004.For example, a multi-level source 1100 is illustrated in FIG. 11 ashaving one or more sub-arrays. The first sub-array 1102 has a float 1106configured to float at the water surface 1108 or underwater at apredetermined depth. Plural source elements 1110 a-d are suspended fromfloat 1106 in a known manner. A first source element 1110 a may besuspended closest to head 1106 a of float 1106, at a first depth z1. Asecond source element 1110 b may be suspended next, at a second depthz2, different from z1. A third source element 1110 c may be suspendednext, at a third depth z3, different from z1 and z2, and so on. FIG. 11shows, for simplicity, only four source elements 1110 a-d, but an actualimplementation may have any desired number of source points. In oneapplication, because the source elements are distributed at differentdepths, the source elements at the different depths are notsimultaneously activated. In other words, the source array issynchronized, i.e., a deeper source element is activated later in time(e.g., 2 ms for 3 m depth difference when the speed of sound in water is1,500 m/s) such that corresponding sound signals produced by the pluralsource elements coalesce, and thus, the overall sound signal produced bythe source array appears as being a single sound signal. In oneembodiment, the high-frequency source elements are simultaneouslyactivated in a flip-flop mode with the source elements of thelow-frequency source elements. In another embodiment, all the sourceelements are simultaneously activated with incoherent, coded signals sothat the recorded seismic signals can be separated based on the sourceelement that emitted that signal.

The depths z1 to z4 of the source elements of the first sub-array 1102may obey various relationships. In one application, the source elements'depths increase from the head toward the tail of the float, i.e.,z1<z2<z3<z4. In another application, the source elements' depthsdecrease from the head to the tail of the float. In another application,the source elements are slanted, i.e., provided on an imaginary line1114. In still another application, line 1114 is straight. In yetanother application, line 1114 is curved, e.g., part of a parabola,circle, hyperbola, etc. In one application, the depth of the firstsource element for sub-array 1102 is about 5 m and the greatest depth ofthe last source element is about 8 m. In a variation of this embodiment,the depth range is between 8.5 and 10.5 m or between 11 and 14 m. Inanother variation of this embodiment, when line 1114 is straight, thedepths of the source elements increase by 0.5 m from one source elementto an adjacent source element. Those skilled in the art would recognizethat these ranges are exemplary and these numbers may vary from surveyto survey. A common feature of all these embodiments is that the sourceelements have variable depths so a single sub-array exhibitsmultiple-level source elements.

The above embodiments were discussed without specifying the type ofseismic receivers used to record seismic data. In this sense, it isknown in the art to use, for a marine seismic survey, streamers towed byone or more vessels, and the streamers include seismic receivers. Thestreamers may be horizontal, slanted or have a curved profile asillustrated in FIG. 12.

Curved streamer 1200 of FIG. 12 includes a body 1202 having apredetermined length, plural detectors 1204 provided along the body, andplural birds 1206 provided along the body for maintaining the selectedcurved profile. The streamer is configured to flow underwater when towedsuch that the plural detectors are distributed along the curved profile.The curved profile may be described by a parameterized curve, e.g., acurve described by (i) a depth z₀ of a first detector (measured from thewater surface 1212), (ii) a slope s₀ of a first portion T of the bodywith an axis 1214 parallel with the water surface 1212, and (iii) apredetermined horizontal distance h_(c) between the first detector andan end of the curved profile. Note that not the entire streamer has tohave the curved profile. In other words, the curved profile should notbe construed to always apply to the entire length of the streamer. Whilethis situation is possible, the curved profile may be applied only to aportion 1208 of the streamer. In other words, the streamer may have (i)only a portion 1208 with the curved profile or (ii) a portion 1208curved and a portion 1210 with a flat profile, with the two portionsattached to each other.

Seismic data generated by the seismic sources discussed above andacquired with the streamers noted in FIG. 12 may be processed in acorresponding processing device for generating a final image of thesurveyed subsurface as discussed now with regard to FIG. 13. Forexample, the seismic data generated with the source elements asdiscussed with regard to FIGS. 3, 7 and 8 may be received in step 1300at the processing device. In step 1302, pre-processing methods areapplied, e.g., demultiple, signature deconvolution, trace summing,motion correction, vibroseis correlation, resampling, etc. In step 1304,the main processing takes place, e.g., deconvolution, amplitudeanalysis, statics determination, common middle point gathering, velocityanalysis, normal-move out correction, muting, trace equalization,stacking, noise rejection, amplitude equalization, etc. In step 1306,final or post-processing methods are applied, e.g. migration, waveletprocessing, seismic attribute estimation, inversion, etc. and in step1308 the final image of the subsurface is generated.

An example of a representative processing device capable of carrying outoperations in accordance with the embodiments discussed above isillustrated in FIG. 14. Hardware, firmware, software or a combinationthereof may be used to perform the various steps and operationsdescribed herein. The processing device 1400 of FIG. 14 is an exemplarycomputing structure that may be used in connection with such a system,and it may implement any of the processes and methods discussed above orcombinations of them.

The exemplary processing device 1400 suitable for performing theactivities described in the exemplary embodiments may include server1401. Such a server 1401 may include a central processor unit (CPU) 1402coupled to a random access memory (RAM) 1404 and to a read-only memory(ROM) 1406. The ROM 1406 may also be other types of storage media tostore programs, such as programmable ROM (PROM), erasable PROM (EPROM),etc. Processor 1402 may communicate with other internal and externalcomponents through input/output (I/O) circuitry 1408 and bussing 1410,to provide control signals and the like. For example, processor 1402 maycommunicate with the sensors, electro-magnetic actuator system and/orthe pressure mechanism of each source element. Processor 1402 carriesout a variety of functions as are known in the art, as dictated bysoftware and/or firmware instructions.

Server 1401 may also include one or more data storage devices, includingdisk drives 1412, CD-ROM drives 1414, and other hardware capable ofreading and/or storing information, such as a DVD, etc. In oneembodiment, software for carrying out the above-discussed steps may bestored and distributed on a CD-ROM 1416, removable media 1418 or otherform of media capable of storing information. The storage media may beinserted into, and read by, devices such as the CD-ROM drive 1414, diskdrive 1412, etc. Server 1401 may be coupled to a display 1420, which maybe any type of known display or presentation screen, such as LCD, plasmadisplays, cathode ray tubes (CRT), etc. A user input interface 1422 isprovided, including one or more user interface mechanisms such as amouse, keyboard, microphone, touch pad, touch screen, voice-recognitionsystem, etc.

Server 1401 may be coupled to other computing devices, such as theequipment of a vessel, via a network. The server may be part of a largernetwork configuration as in a global area network (GAN) such as theInternet 1428, which allows ultimate connection to the various landlineand/or mobile client/watcher devices.

As also will be appreciated by one skilled in the art, the exemplaryembodiments may be embodied in a wireless communication device, atelecommunication network, as a method or in a computer program product.Accordingly, the exemplary embodiments may take the form of an entirelyhardware embodiment or an embodiment combining hardware and softwareaspects. Further, the exemplary embodiments may take the form of acomputer program product stored on a computer-readable storage mediumhaving computer-readable instructions embodied in the medium. Anysuitable computer-readable medium may be utilized, including hard disks,CD-ROMs, digital versatile discs (DVD), optical storage devices ormagnetic storage devices such a floppy disk or magnetic tape. Othernon-limiting examples of computer-readable media include flash-typememories or other known types of memories.

The disclosed exemplary embodiments provide a source array, seismicvibro-acoustic source element and a means for maintaining a pressurebalance between the interior of a seismic enclosure and the ambientpressure so as to ensure that the acoustic actuator can operate withinits design specifications. It should be understood that this descriptionis not intended to limit the invention. On the contrary, the exemplaryembodiments are intended to cover alternatives, modifications andequivalents, which are included in the spirit and scope of the inventionas defined by the appended claims. Further, in the detailed descriptionof the exemplary embodiments, numerous specific details are set forth inorder to provide a comprehensive understanding of the claimed invention.However, one skilled in the art would understand that variousembodiments may be practiced without such specific details.

Although the features and elements of the present exemplary embodimentsare described in the embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the embodiments or in various combinations with or withoutother features and elements disclosed herein.

This written description uses examples of the subject matter disclosedto enable any person skilled in the art to practice the same, includingmaking and using any devices or systems and performing any incorporatedmethods. The patentable scope of the subject matter is defined by theclaims, and may include other examples that occur to those skilled inthe art. Such other examples are intended to be within the scope of theclaims.

What is claimed is:
 1. A source element for generating seismic waves inwater, the source element comprising: an enclosure having an openingcovered by a piston; an actuator system configured to actuate thepiston, wherein the piston is configured to move relative to theenclosure due to the actuator system to generate the seismic waves; alocal supply accumulator located on the enclosure and fluidlycommunicating with an interior of the enclosure, wherein the localsupply accumulator stores a fluid that is also present inside theenclosure, a pressure of the fluid inside the local supply accumulatorbeing larger than a pressure of the fluid inside the enclosure; a localsupply valve located between the local supply accumulator and theenclosure and configured to control a flow of the fluid from the localsupply accumulator to the interior of the enclosure; and an electroniccontroller configured to control the local supply valve to maintain abalance between the pressure inside the enclosure and an ambientpressure of the enclosure while seismic waves are generated, wherein thesource element is a vibratory source element.
 2. The source element ofclaim 1, further comprising: a local vent accumulator fluidlycommunicating with the interior of the enclosure, wherein the local ventaccumulator stores the fluid, a pressure of the fluid inside the localvent accumulator being smaller than the pressure of the fluid inside theenclosure; and a local vent valve located between the local ventaccumulator and the enclosure and configured to control a flow of thefluid from the enclosure to the local vent accumulator.
 3. The sourceelement of claim 2, wherein the electronic controller is also configuredto control the local vent valve such that the pressure inside theenclosure does not fall relative to the ambient pressure with more thana first preset value and does not exceed the ambient pressure of theenclosure with more than a second preset value while seismic waves aregenerated.
 4. The source element of claim 1, wherein the electroniccontroller uses first and second thresholds such that the pressureinside the enclosure does not fall relative to the ambient pressure withmore than the first threshold and the pressure inside the enclosure doesnot exceed the ambient pressure with more than the second threshold. 5.The source element of claim 4, wherein the first and second thresholdsare equal.
 6. The source element of claim 2, wherein both the localsupply accumulator and the local vent accumulator are located on theenclosure.
 7. The source element of claim 2, wherein both the localsupply accumulator and the local vent accumulator are located on a floatto which the source element is attached.
 8. The source element of claim2, wherein the local vent accumulator fluidly communicates with anambient of the source element.
 9. The source element of claim 1, furthercomprising: a global accumulator in fluid communication with the localsupply accumulator and the global accumulator is located in proximity ofthe enclosure; and a pressure-reducing valve located between the globalaccumulator and the local supply accumulator and configured to reduce ahigh pressure inside the global accumulator to a low pressure presentinside the local supply accumulator.
 10. The source element of claim 9,wherein the global accumulator is not located on a vessel towing theenclosure.
 11. The source element of claim 1, further comprising: aninside pressure sensor configured to measure a pressure inside theenclosure; and an outside pressure sensor configured to measure anambient pressure.
 12. The source element of claim 11, wherein theelectronic controller is further configured to receive the pressureinside the enclosure and the pressure outside the enclosure and tocontrol the local supply valve based on a difference between pressuresinside and outside the enclosure.
 13. The source element of claim 1,further comprising: a position sensor configured to measure a positionof the piston, wherein the electronic controller is further configuredto control the local supply valve based on a deviation of the positionof the piston relative to a pre-calculated position of the piston. 14.The source element of claim 13, wherein the pre-calculated position ofthe piston is determined when the source is not submerged in water. 15.A source sub-array for generating seismic waves in water, the sourcesub-array comprising: a float configured to float in water; and pluralsource elements suspended from the float in the water, wherein a sourceelement includes, an enclosure having an opening covered by a piston, anactuator system configured to actuate the piston, wherein the piston isconfigured to move relative to the enclosure due to the actuator systemto generate the seismic waves, a local supply accumulator located on theenclosure and fluidly communicating with an interior of the enclosure,wherein the local supply accumulator stores a fluid that is also presentinside the enclosure, a pressure of the fluid inside the local supplyaccumulator being larger than a pressure of the fluid inside theenclosure, a local supply valve located between the local supplyaccumulator and the enclosure and configured to control a flow of thefluid from the local supply accumulator to the interior of theenclosure, and an electronic controller configured to control the localsupply valve to maintain a balance between the pressure inside theenclosure and an ambient pressure of the enclosure while seismic wavesare generated, wherein the source element is a vibratory source element.16. The sub-array of claim 15, wherein each source element furthercomprises: a local vent accumulator fluidly communicating with theinterior of the enclosure, wherein the local vent accumulator stores thefluid, a pressure of the fluid inside the local vent accumulator beingsmaller than the pressure of the fluid inside the enclosure; and a localvent valve located between the local vent accumulator and the enclosureand configured to control a flow of the fluid from the enclosure to thelocal vent accumulator.
 17. A source element for generating seismicwaves in water, the source element comprising: an enclosure having anopening; a main piston connected to the enclosure to prevent ambientwater entering the opening; a main actuator system configured to actuatethe main piston, wherein a movement of the main piston due to the mainactuator system generates the seismic waves; a secondary piston locatedinside the enclosure and dividing the enclosure into first chamber andsecond chamber, wherein the first chamber is fluidly isolated from thesecond chamber; a secondary actuation system for actuating the secondarypiston inside the enclosure; an electronic controller electricallyconnected to the secondary actuation system and configured to increaseor decrease a volume of the first chamber by moving the secondary pistonfor maintaining a pressure inside the first chamber balanced with anoutside pressure of the enclosure; a local supply accumulator located onthe enclosure and configured to store a fluid; and a local supply valveconfigured to control a flow of a fluid into the first chamber; whereinthe source element is a vibratory source element.
 18. The source elementof claim 17, further comprising: a vent line fluidly connecting thefirst chamber to the atmosphere.
 19. The source element of claim 17,further comprising: a pump attached to the enclosure and configured tovent out a fluid from inside the enclosure.